Citrobacter freundii produces an 18-amino-acid heat-stable enterotoxin identical to the 18-amino-acid Escherichia coli heat-stable enterotoxin (ST Ia)

We purified and sequenced the heat-stable enterotoxin produced by Citrobacter freundii. The toxin was detected during purification by reaction with monoclonal antibody to Escherichia coli heat-stable enterotoxin. The C. freundii toxin amino acid sequence was identical to that of the 18-amino-acid heat-stable enterotoxin (STa) produced by toxigenic E. coli.